1. Field of the Invention
This invention relates to telecommunication networks. More particularly, the invention relates to systems and methods for restoring operation in an optical network during a network failure.
2. Description of the Prior Art
A service disruption in a telecommunications network may be caused by inoperable communications links, cable cut or failure of equipment at a switching node. When a disruption occurs, the time to restore service depends upon a number of factors, such as (a) the time required to identify the location of the service disruption; (b) the time required to determine alternative routes that avoid the service disruption; and (c) the time required to actually establish such routes. In selecting a new telecommunications route, it is desirable to select the most efficient alternate route, that is, the one generally having the least number of nodes and links.
There are two network restoration schemes known to those skilled in the art which locate a service disruption, identify alternate routes and then establish such routes, in order that a service disruption will minimally affect the telecommunications user.
In the first restoration scheme, a telecommunication network includes a central site capable of establishing alternate routes when a failure occurs. The topology of a sample network consisting of a central restoration site is shown in FIG. 1, and also described generally in U.S. Pat. No. 5,182,744 to J. Askew et al. The central site 210 monitors communication paths for alarm signals from switching nodes. In case an alarm is detected, an alternate routing plan is derived and sent to the individual nodes.
In the second restoration scheme, as disclosed in U.S. Pat. No. 5,173,689 to T. Kusano issued Dec. 22, 1992, the network connections are restored by intelligent switching nodes distributed throughout the network. For example, the topology of a sample network consisting of intelligent switching nodes is shown in FIG. 2. Each node in the network (e.g. node 102 of FIG. 2) has the intelligence to identify a failed path, report it to other nodes of the network, and configure an alternate route.
Both restoration systems are designed to be implemented in wire-based telecommunications networks which have a maximum transmission rate of DS3 (DS3 stands for Digital Signal, third level and equals a transmission rate of approximately 44.736 Mbps). In contrast, the transmission rate of optical networks has been increasing steadily, and may reach 10 Gbps by the year 1996. An optical fiber-based system employing optical amplifiers will eventually carry 200 Gbps of information, equivalent to 3840 DS3's. Meanwhile some telecommunications service providers are proposing to use 32 wavelengths across the optical amplifier's amplification passband for their future network operation. This implies that multiple wavelength operation in the available optical spectrum will be commonplace. While simple wired connections may be easily switched in order to reconfigure a network around a fault, the problem lies in how to perform the same switching functions in a high-bandwidth optical network having space and wavelength multiplexing. Accordingly, there is a need for a telecommunications network restoration system employing high-bandwidth optical cross-connect switches enabling rapid switching of an optical network upon service disruption. Such systems must be able to rapidly identify failed optical fiber connections and devise an alternative routing plan using space and wavelength multiplexing to restore the optical network in real time.